Transparent gas-barrier films formed by depositing an inorganic oxide such as silicon oxide, aluminum oxide or the like by vacuum deposition, sputtering, ion plating, chemical vapor phase epitaxy or the like on a film substrate have become popular in recent years as gas-barrier materials against oxygen, water vapor or the like.
The problem is that since these transparent gas-barrier films are generally formed by depositing an inorganic oxide on the surface of a substrate consisting of a tough, transparent biaxial oriented polyester film, left as is the deposited layer is vulnerable to abrasion during use, and when the film is used as a packaging film, the gas barrier properties may be adversely affected by cracks in the inorganic oxide that occur due to abrasion and stretching during subsequent printing or lamination or during filling with the contents, so adequate gas barrier properties cannot be achieved simply with inorganic oxide deposition.
Methods have been proposed of laminating polyvinyl alcohol or ethylene-vinyl alcohol copolymer with gas barrier properties on a biaxial oriented film substrate (see for example Japanese Patent Application Laid-open No. S60-157830, Claims), or covering a biaxial oriented film substrate with a composition of polyvinyl alcohol and poly(meth)acrylic acid (see for example Japanese Patent Publication 3203287, Claim 1). However, the oxygen barrier properties of a gas-barrier film formed by laminating polyvinyl alcohol are lower under highly humid conditions, while a composition of polyvinyl alcohol and poly(meth)acrylic acid has problems of productivity because it requires long-term heating at high temperatures to promote esterification and improve the gas barrier properties of the film, and its gas barrier properties are also insufficient under humid conditions. When reacted for a long time at high temperatures, moreover, the film discolors, detracting from its appearance, so improvements are required for purposes of food packaging.
Since a composition of polyvinyl alcohol and poly(meth)acrylic acid requires a long-term reaction at high temperatures to achieve esterification, methods have been proposed of adding an isocyanate compound or other crosslinking component to the polyacrylic acid (see for example Japanese Patent Application Laid-open No. 2001-310425, Claim 1, Example 1) or further reacting it with metal ions (see for example Japanese Patent Application Laid-open No. 2003-171419, Claim 1, Table 1), but even these methods require high-temperature treatment for 5 minutes at 180 to 200° C. as described in the Examples in order to crosslink the polyacrylic acid with the crosslinking component.
Other means that have been proposed for solving the problems of prior art include a method of laminating polyvinyl alcohol with gas barrier properties on a metal oxide thin film (see for example Japanese Patent Application Laid-open No. H6-316025, Claim 1), a laminated film comprising a deposited layer of an inorganic compound coated on the surface with a coating agent consisting mainly of a an aqueous solution or mixed water/alcohol solution of a water-soluble polymer and at least one of (a) one or more alkoxides and/or hydrolysates thereof and (b) tin chloride (Japanese Patent Publication 2790054, Claim 1), a laminated film obtained by applying a coating composition consisting of a specific organosilane, a silyl group-containing fluorine polymer and an organopolysiloxane (Japanese Patent Application Laid-open No. 2000-63752, Claim 7, Claim 11), and a gas barrier coating film obtained by applying a coating agent comprising a polyvinyl alcohol resin and a metal alcoholate (Japanese Patent Application Laid-open No. 2002-173631, Claim 1, Claim 11). However, gas barrier films obtained by laminating polyvinyl alcohol may have lower oxygen barrier properties under high humidity conditions, while a composition of ethylene-vinyl alcohol copolymer and poly(meth)acrylic acid requires high-temperature treatment for 5 minutes at 180° C. to 200° C. in order to obtain barrier properties under high humidity conditions.